Check how much ice a sawdust layer saves with a heat balance: melting 4,500 kg of ice, the 5 cubic metres a small community needs, absorbs 1.50 gigajoules, so over a 183-day summer the stack can leak no more than about 95 watts on average. Conduction through 15-20 cm of dry sawdust (conductivity assumed 0.06-0.10 W/m.K) around a 5 cubic metre stack with a 10 K temperature difference leaks 53-117 W, so a layer that thin is marginal and 30-45 cm of sawdust is the safer build.
Part of Ice Houses and Cold Storage
Sawdust layering is the placing of sawdust between and around ice blocks. This page tests the thickness rule with the arithmetic of heat flow and latent heat.
The Rule Of Thumb Can Be Checked, And The Check Is Not Comfortable
The parent article says 15-20 cm of dry sawdust around all sides and between layers reduces melting by 80-90 percent compared with bare ice, and that a community of 20-30 people needs roughly 5-10 cubic metres for a full summer. Whether a given thickness lasts the summer is a question with a numerical answer: the energy the ice absorbs in melting against the heat the shell lets through in 183 days. The method below gives it, with every assumption labelled.
What You Need
- The mass of ice — volume x 900 kg/m3, the figure used in the parent article.
- The latent heat of fusion of ice — 334 kJ/kg, the standard value (also stated in Ice Cellar).
- The surface area of the stack and the thickness of the sawdust.
- Sawdust conductivity k — in W/m.K. This is the weak input. Dry loose sawdust is a poor conductor, and handbook values lie in the order of 0.06-0.10 W/m.K. No table was available when this page was written, so the range is an assumption bracket; the conclusions below should be read across the whole bracket, not at one value. Liquid water conducts about 0.6 W/m.K (see Thermal Insulation Basics), so damp sawdust is far worse.
- The temperature difference between the ice (0 degrees C) and its surroundings. The parent article gives the ground at 10-15 degrees C, so the difference is 10-15 K.
- The season length — 183 days, April to September.
Method: The Heat Balance
Step 1: Energy needed to melt the ice. E = m x L, with L = 334,000 J/kg.
Step 2: Average leak the ice can survive. P_max = E / t, with t in seconds.
Step 3: Surface area of the stack. Take a cube for the best case. Side = V^(1/3); area = 6 x side^2. Real stacks have more surface for the same volume.
Step 4: Heat leak through the shell. By Fourier’s law for a flat layer, Q = k x A x dT / L, with L the sawdust thickness. This treats the sawdust as a flat slab round a cube. It underestimates the area at the corners (the true outer surface is larger) and ignores the doors and ventilation.
Step 5: Fraction melted. Q x t / E.
Worked Example: A Small Community’s Stack
Budget.
- Mass: 5 x 900 = 4,500 kg.
- Energy: 4,500 x 334,000 = 1.503 x 10^9 J, or 1.50 GJ.
- Time: 183 x 86,400 = 15.81 x 10^6 s.
- Average leak allowed: 1.503 x 10^9 / 15.81 x 10^6 = 95 W.
Shell.
- Cube side: 5^(1/3) = 1.71 m. Area: 6 x 1.71^2 = 17.5 m2 (the stack’s outside surface, ignoring the thickness of sawdust).
One case in full. Sawdust 0.20 m thick, k = 0.08 W/m.K, dT = 10 K.
- Q = 0.08 / 0.20 x 17.5 x 10 = 0.4 x 175 = 70 W.
- Energy lost: 70 x 15.81 x 10^6 = 1.11 x 10^9 J.
- Ice melted: 1.11 x 10^9 / 334,000 = 3,320 kg, which is 3,320 / 4,500 = 74 percent.
Across the bracket (5 m3, dT = 10 K, 183 days; melted share of the ice):
| Sawdust thickness | k = 0.06 | k = 0.08 | k = 0.10 |
|---|---|---|---|
| 15 cm | 74% | 98% | 123% |
| 20 cm | 55% | 74% | 92% |
| 30 cm | 37% | 49% | 62% |
| 45 cm | 25% | 33% | 41% |
Across the 15-20 cm rows the leak runs from 53 W (20 cm, k = 0.06) to 117 W (15 cm, k = 0.10), against the 95 W budget. A value above 100 percent means the ice is gone before the season ends. With dT = 15 K, every figure is 1.5 times larger (20 cm at k = 0.08 becomes 111 percent). A 10 m3 stack has a smaller surface per cubic metre (27.9 m2 for 10 m3, or 2.8 m2/m3, against 3.5 for 5 m3), so at 20 cm and dT = 10 K it melts 44-73 percent. Warm air at the roof is worse than ground at the sides; with the top at dT = 20 K and the other faces at 10 K, the 20 cm, k = 0.08 case reaches 86 percent. Damp sawdust at k = 0.2, a 20 cm layer and dT = 10 K melts 185 percent.
What This Does And Does Not Show
- Thickness matters more than the label. In this model, the 15-20 cm the parent article gives is marginal for the small stack and only comfortable at the cool, low-k end. Storage Buildings Design gives 30-50 cm and Ice Cellar gives 30-45 cm; the table above supports the thicker figures.
- The layer between blocks is not a conduction barrier. All the heat must cross the outer shell, so sawdust between layers does not add to the shell’s resistance. Its role is to stop the blocks fusing, to fill gaps that would let air circulate and to take up meltwater. The parent article’s “80-90 percent” reduction relative to bare ice is not something this model verifies.
- Doors, ventilation and warm air entering are not counted, and each opening adds to the leak.
- The ground is not exactly 10-15 K warmer than the ice, and the stack leaks heat through the floor only if the gravel and drain keep it dry.
- Instrument it. Meltwater is a direct reading of the heat leak. The heat leak is the mass melted per day x 334,000 / 86,400. A 95 W leak melts 95 x 86,400 / 334,000 = 24.6 kg a day. If the drain delivers 47 litres a day (as in the example in Meltwater Drainage) the leak is 47 x 334,000 / 86,400 = 182 W. Catch the drain water in a bucket for a day and you have the number.
Common Mistakes
- Reading the table as a prediction — k is an assumed range and the shell is idealised.
- Counting sawdust between layers as extra shell — it is not on the heat path.
- Using damp sawdust — the k = 0.2 line shows the result.
- Forgetting the roof — the top sees the largest temperature difference.
- Skipping the measurement — a bucket under the drain replaces a lot of guessing.
Safety
Thicker sawdust is more fuel and, in a closed pit, less air
A pit lined and capped with 30-45 cm of dry sawdust holds a great deal of fuel. Keep flames and hot ashes away. Damp sawdust uses up oxygen and releases carbon dioxide that pools at the bottom of a closed pit. Never descend until it is ventilated to the floor, tether the person who goes in, keep an attendant at the top, and never test the air with a flame. See Ice House Construction.
What’s Next
- Learn next: Air Gap Prevention — the part of the leak sawdust cannot stop
- Combine with: Sawdust — sourcing and drying the material